The decomposition of neutron-induced gamma ray spectra using full-energy reconstruction based on a linear combination of elemental standard spectra has been applied in many industries. Normally these elemental standard spectra are measured in a controlled environment that may not be representative of the environment to which they are applied. Anything in the application environment that significantly alters the effects of gamma ray scattering may make the elemental standards derived from the more simplistic environment inappropriate for the application environment. For example, changes in density have an impact on gamma ray scattering, and changes in hydrogen concentration may change the characteristic distribution of neutrons that defines the gamma ray source.
For oil well logging inside a borehole cased with a metal (e.g., steel) casing and/or cement, the additional gamma ray scattering from both the metal casing and cement reduces the total number of detected gamma rays. Moreover, this also changes the spectral character of the detected gamma ray spectra since the effects of scattering are very dependent on the energies of the gamma rays. It is possible to measure an energy spectrum for a given geological constituent(s) in a reference (e.g., open-hole) environment, and then transform the measured spectrum to account for the changes in scattering (e.g., from a casing, etc.). However, to extrapolate such measurements to a different borehole environment (e.g., a cased borehole) would ordinarily warrant using Monte Carlo modeling to follow the entire history of each of the hundreds of gamma ray lines of different energies and intensities that contribute to the final spectrum, which may not be practical or even possible in many applications.